1. Field of the Invention
The present invention relates to vehicle steering systems, and more particularly to vehicle four wheel steering systems wherein rear wheels as well as front wheels are steered.
2. Description of the Prior Art
Conventionally, a four-wheeled vehicle has a steering system which is adapted to steer only the front wheels. It has however been proposed for the purpose of improving the maneuverability of the vehicle to steer not only the front wheels but also the rear wheels. It has further been proposed in such four-wheel steering system to provide a power-assisted steering mechanism for steering the rear wheels. For example, the U.S. Pat. No. 4,467,885 discloses a vehicle four-wheel steering system having a power-assisted steering mechanism provided for steering the rear wheels.
In general, a power-assisted steering mechanism includes a power cylinder and a power piston disposed in the power cylinder for axial sliding movement to define hydraulic oil chambers in the power cylinder. Further, a control valve mechanism is associated with the power cylinder to control the hydraulic fluid flow into and out of the hydraulic chambers in the power cylinders. It should be noted herein that such power-assisted steering system has a lot of conduit connecting the hydraulic chambers in the power cylinder with the control valve, and connecting the control valve with the hydraulic pump and the hydraulic oil reservoir. Usually, these conduits are provided by external pipes and hoses so that a lot of pipes and hoses have to be arranged in a limited space. Particularly, in a power-assisted rear wheel steering mechanism, available spaces for the conduits are very limited so that it is very difficult to arrange the conduits in a reliable manner.